The present invention relates to a method and apparatus for the confidential transmission of voice signals by encoding such signals at a transmitter facility and by decoding the encoded signals at a receiver facility.
In the confidential transmission of voice signals, a reduction of the level of confidentiality may occur by the presence of breaks or interruptions of the voice signals. The breaks appearing between individual syllables or words of the audio signal are not masked by present day coding methods, for example alternating the interchange of individual partial frequency bands, and as a result the voice signal portion of transmission can be recognized within the coded signal. Assumptions regarding the spoken text can be derived from the duration and frequency of the breaks. The length and exact position of an individual spoken word can be exactly determined and an unauthorized person may then concentrate on any additional recognition methods on this isolated word in order to decode it as quickly as possible.
Even through the use of more effective coding methods, wherein voice signals are decomposed into scanning values whose coding is effected, for example, by amplitude variation corresponding to alternating code signals, breaks and interruptions can still have a harmful effect since the coded signals corresponding to the signal gaps permit direct conclusions upon the respective code signals used, whose recognition is undesirable in view of possible insight into the principles and techniques employed into the production of the coded signals.